Fluid-borne noise is commonly present in hydraulic systems powered by pumping apparatus such as gear, vane or piston pumps. Typically, the noise results when pressure waves generated as pump flow ripple encounter system flow impedance. Fluid-borne noise present in a hydraulic system causes mechanical apparatus, such as hydraulic lines, control valves, hydraulic motors, and supporting structural members, to vibrate. In many cases, such vibration is coupled to the atmosphere and is the source of objectionable acoustic noise. It is of course, desirable to attenuate such fluid-borne noise.
Fluid-borne noise reduction apparatus in the high pressure side of the prior art power steering system usually comprises a flexible metal or plastic tube, called a tuning cable, placed inside a section of volumetrically compliant hose. One of the earliest such prior art apparatus is that disclosed in Klees U.S. Pat. No. 3,323,305.
In the low pressure side of prior art power steering systems, another type of “tuning”0 device is used, one that does not employ the tube-within-tube tuning cable concept but rather simply a restrictor in the return hose line. Such restrictors usually are of the constant-diameter-passageway type shown as restrictor 10 in Katayama et al. U.S. Pat. No. 4,285,534. Such restrictor elements are inserted in a selected location in the low pressure return line flexible hose to form a flow barrier that helps “balance”0 pressures in the power steering system and thereby prevent “shudder”0 under certain operating conditions, as is well understood in this art.
In many power steering systems, such a balancing restrictor inserted in the return side of the circuit typically operates under system fluid pressures of approximately 100 to 150 psi, which are much lower pressures than are typical in the pump output side of the power steering circuit where pressures may be in the vicinity of 1500 psi. In accordance with the present invention, it was noted that under certain conditions the return side restrictor produced an audible “hiss” noise that could be heard in the passenger cab. It was speculated that the cause might be the abrupt internal angle (chamfer) at the entrance to the flow-through, constant diameter passage of the restrictor, the abrupt angle at the exit of this flow-through passage, the surface finish of the passage, the possibly sharp edges at the entrance and exit of the flow-through passage, as well as the abrupt diameter differential between the hose I.D. and the restrictor passage I.D. Any or all of these parameters were thought to have contributed to making the fluid flow go turbulent in the vicinity of the exit of the restrictor flow-through passage, thereby producing the “hiss” noise.
Another problem, unrelated to the noise problem experienced with the current production low pressure side restrictor (often referred to in the trade as a “dogbone”), was hose pinching when the crimp collar location was not correct relative to the dogbone exterior shape of the restrictor. In such cases, the raised edge of the restrictor sometimes caused hose damage and failure.
Assuming that the “hiss” noise problem was indeed due to the creation of a turbulence condition in the fluid flow just before or after the exit of the restrictor, ancillary problems would be excessive pressure drop and heat generated by such turbulence, as well as potential cavitation wear on the wall of the tubing. Moreover, these ancillary problems could be present even in the absence of the turbulence reaching a level sufficient to produce the annoying audible “hiss” sound.
Accordingly, and by way of summary description, and not by way of limitation, the present invention is directed to providing a new form of restrictor for use in noise-reduction fluid conduits subject to pressure pulsations. The restrictor has a central flow-through passage open at its opposite axial ends, but this flow-through passage is configured in the form of a classic venturi instead of the conventional constant diameter flow-through passage. In a present preferred but exemplary embodiment of the invention, the restrictor has its flow-through passage configured as a venturi having a shallow convergent (in the direction of fluid flow) tapered inlet, a constant diameter throat and a shallow divergent (in direction of fluid flow) tapered outlet. The restrictor flow-through passage is thus constructed with the configuration of a venturi inlet, throat and outlet arranged and operable, under the pressure and fluid flow conditions in which the restrictor is used, so as to minimize turbulence in the restrictor outlet and/or immediately downstream thereof.
Preferably the cross sectional configuration of the interior of the restrictor is symmetrical about all axes, and the taper angle for both the inlet and the outlet is approximately 8°. Although the restrictor can be made of metallic material such as brass, aluminum or steel, it is preferred to injection mold the restrictor out of a suitable plastic material to achieve smoother wall surfaces in the flow-through passage, as well as for economy of system assembly and part costs.
In accordance with the method of the invention, the venturi inlet, throat and outlet are designed so as to conduct fluid therethrough in the operating system of the pressure fluid device by matching the characteristics of the fluid, the operational pressures, fluid density and other relevant system parameters, such that the venturi operates below the lower critical value of the Reynolds number of the fluid flow through the restrictor to thereby minimize or eliminate noise by minimizing or eliminating turbulence in the fluid in the restrictor outlet and/or exiting immediately downstream from the venturi restrictor. As indicated hereinabove, the venturi restrictor of the invention is presently intended primarily for use in the power steering return line to the system reservoir that supplies the pump input side of the power steering circuit where fluid pressures of 100 to 150 psi are typical, because this appears to be the area where the turbulence problems are most acute and are manifesting themselves with the annoying “hiss” noise. However, the preferred embodiment of the restrictor, due to its other advantageous features as well as the venturi shape of the flow-through passage, may also be advantageously employed in the high pressure side of the power steering circuit as a central restrictor in the typical tuning cable assembly employed in this side of the system.
Other and ancillary novel features of the invention will become apparent from the following detailed description and appended claims.